In spectrometry the spectral components emitted by bodies and substances are observed using a spectroscope. The spectral components can be separated from one another on a frequency basis. This separation can take place using a grating in which a series of very fine grooves are used to disperse incident electromagnetic energy.
Standard concave grating spectrometers admit electromagnetic energy in the form of light from a source through a small aperture and focus the spectrum on a suitable surface. For standard ruled concave gratings, the surface is called a "Rowland Circle". The surface is a flat focal plane in the case of flat-field holographic, stigmatic holographic or specially ruled concave gratings.
Standard plane grating spectrometers also admit light from a source through a small aperture. A mirror or lens then collimates the light, which illuminates a plane grating that initially disperses the spectrum largely onto a focusing mirror, followed by focusing onto a flat focal plane. A single small aperture may then be used with a single detector and the grating turned in order to sequentially produce monochromatic light of varying wave lengths at the aperture.
Conventionally, small apertures and single detectors are placed on the Rowland Circle for specific wavelengths of interest, or for flat-field holographic gratings, array detectors with many closely spaced detectors are placed on a flat focal plane. The array detectors allow the whole spectral range of interest to be observed simultaneously through electronic multiplexing. These array detectors may also be used with plane grating spectrometers, but will generally cover only small segments of the total spectrum.
Array detectors have found to be advantageous in applied spectroscopy. They allow rapid acquisition of the complete spectrum, since they employ parallel rather than sequential data acquisition. In addition, there often is an enhanced signal-to-noise advantage with multiplexing. Array detectors also can eliminate the need for moving parts in the spectrometer system, resulting in reduced cost and improved life and durability.
Array detector technology for the visible region of the spectrum has advanced rapidly because of the demand for similar types of devices created by telecommunications. Such detectors have been based on silicon light detection over the spectrum ranging from deep ultraviolet (200 nm wavelength) to the close near infrared (1100 nm wavelength). Silicon light detectors perform well and are relatively inexpensive with a cost ranging from tens to hundreds of dollars.
Array detectors which are useful in the near and mid-infrared region of the spectrum have been developed for military use in smart weapons. While military arrays would be useful for general spectroscopy, their complexity makes them unsuitable for mass manufacturing. The spectroscopes that have been designed on the basis of military technology are low quality and are extremely expensive, each costing thousands to tens of thousands of dollars.
Accordingly, it is an object of the invention to facilitate spectroscopy. A related object is to facilitate spectroscopy in the frequency ranges where conventional methods have proved to be inadequate or too costly.
A further object of the invention is to eliminate the need for array detectors in spectroscopy, while realizing the advantages of array detection.
Another object of the invention is to avoid the need for complex gratings in facilitating spectroscopy where conventional methods have proved inadequate. A related object is to use plane grating spectrometers to achieve performance comparable to more complex spectrometers. A further object is to use plane grating spectrometers, without the need for array detectors and the disadvantage of such detectors in generally covering only small segments of the total spectrum.